The ban on gay marriage is one of the disputes dividing the Democratic Unionists and Sinn Fein as they try to hammer out a deal to end the 13-month political crisis.

Sinn Fein wants a law change while the socially conservative DUP is firmly opposed to any redefinition of the marriage law.

A majority of MLAs were in favour of lifting the ban when the issue last came to the floor of the Assembly chamber, but the DUP triggered a contentious voting mechanism - the petition of concern - to block it.

Senior figures from Sinn Fein, the SDLP, the Ulster Unionists, Alliance, Greens and People Before Profit posed for photos with the campaigners in Parliament Buildings, Belfast on Wednesday morning.

Social Democratic and Labour Party leader Colum Eastwood, at Stormont Parliament Buildings during a same sex marriage event

The presence of UUP representatives Doug Beattie and John Stewart demonstrated that the debate does not fit into Northern Ireland's traditional orange and green prism.

Other UUP members oppose a law change and could potentially join forces with the DUP to trigger another petition of concern in the future.

If 30 or more MLAs sign a petition then any subsequent Assembly vote on a proposal must have the support of a majority of unionist and a majority of nationalist members, rather than a straight head count majority.

The activists want Mrs Bradley to intervene and introduce a law change through Westminster if there is no resolution to the stand-off at Stormont.

Jennifer McAlpin, a 16-year-old student from Ballycarry, Co Antrim, was one of those who brought the petition to Stormont.

"We chose Valentine's Day because it is the day of love, so equal love," she said.

"It's important to me because I am a young gay teen in Northern Ireland and I don't want to be prejudiced (against) as I grow up, I don't want my friends to be prejudiced (against) as they grow up."

Mr Beattie said the campaign for same sex marriage was "right and just".

He stressed the issue was a matter of conscience within the UUP and colleagues with opposing views should not be denigrated.

"I have always support marriage equality and I have been very vocal on that," he said.

"I think it's important to be here today to show that support and that solidarity for those who want that marriage equality."

SDLP leader Colum Eastwood said a reform of the petition of concern was an obvious way to make progress on same sex marriage legislation.

"We should allow the majority of the Assembly's voice to be heard and the majority of the community's voice to be heard in making sure that everybody on this island is entitled to have their love fully respected and be married if they so wish," he said.

While the marriage issue does feature in the long-running political negotiations, the main obstacle to the restoration of powersharing remains the Irish language.

The DUP and Sinn Fein negotiating teams are set for crunch meetings on Wednesday to establish if the basis for an agreement exists.

Weekend optimism that a deal could be close appeared to recede on Tuesday after the impasse over the Irish language deepened.

Both parties set out public positions on Tuesday that suggested a meeting of minds on the crucial Stormont sticking point was still a long way off.

DUP leader Arlene Foster made clear she would not sign off on a stand-alone Irish Language Act - a key Sinn Fein demand throughout the 13 months Northern Ireland has been without devolved government - and also ruled out any legislation that would see gaelic road signs or workforce quotas of Irish speakers within the civil service.

Sinn Fein insisted an Irish Language Act was a prerequisite of any deal.

Party president Mary Lou McDonald told the DUP to show leadership and ignore the hard-line critics who were opposed to any settlement.